Modified carbide fuels



Oct. 17, 1967 K. R. JORDAN 3347349 MODIFIED CARBIDE FUELS Filed Sept. 7, 1965 2 Sheets-Sheet l UC+CUBIC UCZ TEMPERATURE C TETRAGONAL FIG. 2

4 FIG. I.

wITNEssEs: INVENTOR gv@ @f/'1717 Kenneth R. Jordon 06f. 17, 1967 K R JORDAN MODIFIED CARBIDE FUELS 2 sheets-sheet 2 Filed Sept. '7,'1965 (4.56m B (5.o ucr IO ATOMIC PERCENT CHROMIUM United States Patent fi dee 3,347,749 MODIFIED CARBIDE FUELS Kenneth R. Jordan, Monroeville, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Sept. 7, 1965, Ser. No. 485,374 25 Claims. (Cl. 176-69) This invention relates to nuclear fuel material and more particularly, it pertains -to carbides of uranium, plutonium, thorium, and solid solution compounds thereof.

Uranium carbide and mixed carbides, such as (UPu)C, (ThPu)C, and (ThU)C, have several attractive features which recommend their use in power reactors. The high density of heavy metal atoms is desirable for breeder applications and the relatively high thermal con-ductivity of carbide fuels permits operation at high power densities. In order to utilize the latter advantage, the fuel is preferably bonded to an outer cladding tube of a fuel velement by a liquid metal such as sodium or a sodium-potassium alloy (NaK).

vOne disadvantage of carbide fuels is the potential problem of incompatibility between the fuel an-d the cladding tube. For many applications, stainless steel is the preferred material for the cladding tubes. lDue to ythe limitations of current fuel synthesis techniques, the fuel composition cannot be controlled sufficiently well to yield a stable single phase product. Small'deviations of carbon content from stoichiometric proportions result in -a product that contains either free metal or higher carbides, such as MC2, or M2C3, where M is a heavy metal atom such as uranium, plutonium, or combinations lthereof. For example, a stoichiometric carbide consists of UC; however, a hypostoichiometric carbide (which has a deficiency of carbon) consists of a mixture of U and UC. A hyperstoichiometric carbide (which contains too much carbon) consists of UC and UC2. Similar examples can be cited for other elements including uranium and plutonium, which form solid solution compounds with the carbon.

With uranium carbide having less than 4.8 w/o carbon, there is an excess of free me-tal uranium in the matrix which can form an eutectic with the stainless steel cladding which melts at 1338 F. which is too low or marginal at best for satisfactory operation in a fuel element. With uranium carbide having more than 4.8 w/o carbon, uranium dicarbide (UC2) forms which is attacked by the sodium. The reaction is as follows:

where Na(C) denotes sodium containing carbon in solution.

Where the higher carbides are present in a fuel which is so-dium bonded to stainless steel cladding tubes, the carbon in the fuel transfers through the sodium to the cladding material above about 1000 F. and consequent cladding embrittlement is observed. Thus, premature failure of 4the stainless steel cladding will occur. Where, however, free metal such as uranium exists, that is, where there is a deficiency of carbon, low melting eutectics with the stainless steel are formed. Compatibility of the fuel and the cladding therefore must be assured whereby there is no detrimental reaction, before the full economic and technical advantage of carbide fuels can be realized.

Prior attempts to develop carbide fuels which exhibit satisfactory compatibility with the cladding have generally been directed toward increasing the range of carbon contents over which the single phase can be synthesized. That is usually attempted by adding alloying elements such as zirconium which themselves form monocarbides that are stable over a range of compositions. Heretofore, no system has been identified which results in a single nate the operational limitations imposed by problems of fuel-cladding incompatibility of both hypostoichiometric (carbon deficient) and hyperstoichiometric (carbon excess) fuels and result in improved irradiation stability for the former. Secondary phases are provided in a given fuel element by the addition of suitable alloying additions. The amount and the nature of the addition are dictated to insure that phases are obtained that will not give up carbon to sodium or NaK or form low melting eutectics with stainless steel. That is, the presence of MC2, M2C3, and M phases are avoided.

Accordingly, it is a principal object of this invention to provide a modified carbide nuclear fuel material which obviates or minimizes prior known difiiculties.

It is another object of this invention -to provide a modified carbide nuclear fuel material which avoids prior disadvantages of cladding embrittlement due to the migration of carbon from the fuel to the surrounding cladding tube.

It is a further object of this invention to provide a modified carbide nuclear fuel embodying predetermined proportion of chromium which avoids the incompatibility problem of forming low mel-ting eutectics-with stainless steel cladding tubes.

Other objects and advantages of the invention will be-A tailed description and to the drawing in which: Y

FIGURE 1 is a longitudinal sectional view through an elongated fuel element;

FIGURE 2 is a transverse sectional View taken on the line II-II of FIG. 1;

FIGURE 3 is a constitutional diagram of a part of the uranium-carbon system;

FIGURE 4 is a portion of the constitutional diagram of the ternary system of uranium, carbon, and chromium in atomic percentages, and showing weight percentages in parentheses; and

FIGURE 5 is an enlarged portion of a part of the diagram of FIG. 4.

Briefly, the present invention comprises the alloy of a nuclear metal carbide, such as uranium, plutonium or uranium-plutonium carbide, with small predetermined proportions of chromium or chromium carbide, or both. The chromium forms stable carbides, particularly Cr23C6, with any excess carbon. The resulting chromium-nuclear carbide alloys are compatible with stainless steel cladding with liquid metal bonding. The alloys are highly effective nuclear fuels and fuel elements made therefrom have a long life. Since small traces of oxygen and nitrogen are present, their effects are similar to that of carbon on an oxygen and nitrogen, or on a an equal number of carbon the total atoms of carbon, weight basis, converted to atoms.

More specifically, the invention comprises an improved nuclear fuel element comprising a stainless steel cladding or sheath, a body of fuel comprising an alloy of 'MC and chromium carbide with or without any chromium, and a liquid metal such as sodium or NaK disposed in the cladding and filling the space around the body of fuel.

As a further specific embodiment of the present invention, a three-phase uranium base alloy with carbon and chromium is provided as is shown within the triangular area A-B-Y of FIG. 4. Though it is preferred to use as little chromium as possible in the alloy, there may be as much as about 5 w/o (11.125 atomic percent) chromium,

Patented Octlv'?, 1967 where w/o is weight percent. Typical samples of the alloy may vary from 0.40 to 5.17 w/o chromium. For alloys having 0.40 w/o chromium, there may be from 4.78 to 4.8 w/o carbon and for alloys having fro-m 5.01 to 5.17 w/o chromium the carbon may vary, respectively, from 4.56 to 4.85 w/o. For alloys having from 3.32 to 3.48 w/o chromium, the carbon content may vary respectively from 4.65 to 4.84 w/o. For plutonium and uranium-plutonium alloys, similar three phase proportions are produced.

The three component alloy constituting the present invention may be used, among other things, as a fuel pellet such as in an elongated fuel element generally indicated in FIG. l. Referring to FIGS. l and 2, the fuel element 1 includes an elongated tube 2 having end caps 3 and 4 attached thereto by similar welds 5. The tube 2 is preferably composed of stainless steel. Within the tube 2, a plurality of fuel pellets 6 are provided in end-to-end abutment within a chamber 7 provided by the tube.

As shown in FIG. l, the diameter of the pellets is slightly less than that of the inner surface of the tube 2 thereby providing clearance of about 6 mils between the tube and the pellets. The clearance is lled with a liquid metal such as sodium or NaK. which is sustained in a substantially stagnant status as a thermal bond.

The pellets 6 constitute the fuel by which a nuclear reactor is operated. Accordingly, the fuel may be composed of fissionable material such as uranium carbide, plutonium carbide, (UPu)C, (UTh)C, (PuTh)C, and the like.

In the past, uranium carbide and uranium plutonium carbide, UC and (UPu)C, have been used as monocarbide fuels. As shown in the phase diagram of FIG. 3 uranium carbide having 4.8 w/o carbon is an intermetallic compound of precise stoichiometric composition. Due to limitations of current synthesis techniques in this area, the binary compound composition cannot be controlled sufficiently well to yield an exact single phase product. Typical deviations of carbon content are about i0.l W/ o carbon resulting in a product that contains significant amounts of higher carbides such as MC2 or M2C3 on the one hand, or free metal M if carbon is deficient.

Because of the lack of control over the precise stoichiometric composition of these carbides, an alloying addition is made to insure the development of a three phase equilibrium composition thereby avoiding the development of the phases which create the previous problems of fuelcladding incompatibility. The preferred alloying element is chromium, because of its ability to combine with carbon to form Cr23C6 which is a stable phase. In addition to the Cr23CG phase, the alloy contains UC and Cr. The amounts of any chromium and/ or chromium carbide addition, however, are not sufficient to affect detrimentally the satisfactory overall operation of a nuclear reactor.

The amount of the addition and the nature of the addition are chosen to insure that phases which will give up carbon to sodium or NaK, or which form low melting eutectics with stainless steel are not found in the carbide alloy fuel. In particular, the presence of MC2, MZCB, and M phases, is avoided, where M is uranium, plutonium or UPu. Such additions take advantage of the fact that at some ternary compositions, MC is in equilibrium with two Cr and Cr23C6 phases which do not adversely effect cornpatibility with the cladding or the irradiation stability of the fuel. At the same time, allowance is made for variation of the carbon content of the fuel pellets.

A portion of the ternary system of uranium, carbon, and chromium is shown in FIG. 4 and an enlarged portion thereof in FIG. 5 in the vicinity of the UC composition to which additions of an alloying element such as chromium are made. As shown in the diagrams, a stoichiometric UC fuel composition contains 4.8 w/o carbon or 50 a/o (atomic percent) carbon. In the practice the supplier of UC produces any composition within i0.1% C. The limiting composition which corresponds to typical pellet-to-pellet variations of $0.1 w/o carbon from 4.9 w/o are shown at 50.0 and 51.05 a/o carbon points on the ternary diagram. Where no chromium addition is present, the fuel will consist of UC and U2C3 phases in equilibrium. However, the addition of about 3.4 w/o chromium is sufficient to insure that no U2C3 will be present in uranium carbide compositions containing carbon in the range specified by such minimum and maximum carbon contents. Thus, a random selection of pellets will exhibit the following structures represented between points X and Z on the ternary diagram: Carbon content: Phases:

(1) Minimum 4.8 w/o UC-Jf-Cr (2) 4.8 w/o C Carbon content 5.0 w/o C UC-i-Cr-l-Cr23C6 (3) Maximum 5.0 w/o C UC-l-CrzgCG As shown in FIGS. 4 and 5 the uranium carbide may contain as little as 0.4 W/o (about 0.95 a/o) chromium or as much as about 5.17 w/o (about 1.25 a/o) chromium. Where the supplier provides a UC fuel having an excess of carbon, such as 4.9 w/o carbon, enough chromium metal is added to bring the final fuel analysis within the triangular area A-B-Y of FIG. 4. Similarly, Where the UC composition provided has a deficiency of carbon, such as 4.7 w/o carbon, chromium carbide (Cr23C6) iS added in sufcient quantity to result in a basic UC analysis that is within the triangular area. In practice however, where the calculated additions of chromium and/ or chromium carbide are made and the mixed powders are sintered to form a fuel pellet the resulting compositions are within the trapezoidal area A-B-D-E. A typical analysis may occur in a zone adjacent the line Z-X. These several points have compositions by weight as follows: j#

Carbon, percent To avoid the formation of free uranium metal, additions of an alloying chromium carbide may be made to obtain a three phase equilibrium that does not contain the undesirable free metal.

As an example, a typical analysis of a uranium carbide powder gives 4.45 W/o carbon with incidental impurities including 0.2 w/o oxygen, 0.01 W/ o nitrogen, 0.03 w/o molybdenum, plus less than 500 p.p.m. of other elements. To this carbide about 3.6 w/o chromium carbide (Cr23C6) powder is added to obtain the desired pellet analysis. Where a pellet composed of uranium-plutoniv um carbide is used, the plutonium may vary from 15 to 20 W/ o of the uranium.

Oxygen and nitrogen have effects on uranium or uranium-plutonium similar to carbon. Thus, the amounts of carbon added are indicated as effective carbon. The effective carbon content is the sum of the weight percentage of carbon, plus adjusted weight percentages of oxygen and nitrogen. For example, for a powder analysis having 4.6 w/o carbon, 0.2 W/o oxygen, and 0.07 w/o nitrogen the effective carbon content is 4.81 equivalent w/o carbon,

calculated as follows:

Actual Analysis Multiplier 1 Carbon Equivalent Equivalent w/o carbon 4. 81

1 Where lthe multipliers are the ratlos `of atomic weights of each element divided into the atomic weight of carbon,

Uranium carbide powder having a hypostoichiometric fer. Modification of carbide fuels to assure compositions which exhibit three phases in equilibrium, Vi.e. UC, Cr23C6, and Cr, rather than attainment of a single Aphase strucana-lysis of 4.6 to 4.8 w/o carbon is mixed with a quan- 5 ture isplqposed as a method of enhancing fuelicladdingr tity of Cr23C6 so that the chromium added as Cr23C6, compatlblhty' constitutes about 3.4 w/o of the total. The powder mix- Moreover. a Speclc System lmely M-C-C'f- 1S aP' ture is pressed into a compact pellet which is then sinpaliently umque m that Composmons an be reahzed m tered for three to four hours at a temperature ranging wh1c-h au Secondary phases are nonssile .Phasfes .and the from 1400 to 1500s C The resulting pellet is approxi 10 modi'ed fuel should therefore:- exhibit irradiation stamately 90 percent dense. A formula for the foregoing blhty comparable to that of gtmchloilemc MC' The aP' chemistry involved for hypostoichiometiic carbon analy- Phroxlmate amounts .)f alloymg additions are Shown m sis of uranium carbide powder of varying carbon pert e table fof the .vallous systems of Interest assuming a centages is as f0110wspellet-to-pellet variation 1n carbon content of 0.1 weight 15 percent about nominal composition. Cr23C6+UC(-4.6 w/o C)- UC-l-Cr.'-Cr23C6(f3e) Finally, the chromium addition not only improves the Cr23C6+UC(-4.7 w/o C)- UC|Cr-ICr23C6 compatibility between the fuel and the cladding but alsoV Cr23C6+UC(-4.8 W/o C)- UC|-Cr(trace)+Cr23C6 results in irradiation stability. There is also an incidental advantage of reduced sintering temperature. thgullrgstggleggralysls of uramum carblde It should be understood that vthe above description is only exemplary and not in limitation of the invention. Cr+UC -48 w/O C)- UC+Cr+Cr23C6(trace) what is claimed is; Cr+UC(-4-9 W/ 0 C) UClC1l-C1`23Ce 1. A ternary alloy consisting essentially by weight of Cr+UC(-5.0 w/o C)' UC+Cr(trace)+Cr23C6 4.56 to 4.85% carbon, 0.40 to 5.17% chromium, and the As shown above, the hypostoichiometric uranium car- 25 balm?? Consisting O f an element Selected from the gfOuP bide is mixed with chromium carbide (Crzscs) for the consisting of uranium metal, plutonium metal, and purpose of adding additional carbon in order to avoid uranum'plutomum alloy the presence of free uranium metal. In other words, the 2- The 13111315 31103 0f 6131111 1 111 WhlCh the uraniumaddition of Cr23C6 to the uranium carbide prior to sin- P1uf0111u111 2110) comprises from l5 to 30% plutonium, tering adds a sufficient amount of carbon to react with all 3- The 16111315 31103/ 0f 0131111 1 111 Which there 1s about free uranium which is not tied up with carbon as uranium 3-4% Chromlum carbide 4. The ternary alloy of claim 1 in which there is about For hyperstoichiometii'c UC, unalloyed chromium is 475% Carbon Y added to avoid the formation of UCZ by forming the 5. The ternary alloy of claim 1 in which there is about more Stable Crzscw 3.4% chromium and about 475% carbon.

CrgSCs or chromium metal may be mixed, respective- 6- The 1113ry alloy of claim 2 m which there 1s about ly, with hypostoichiometric or hyperstoichio-metric 34% Chromlumm-onocarbide fuels, other than uranium carbide, such as 7- The tefuaf Y 3110i/ 0f c131111 2 111 Whlch there is about PuC, or (UPu)C, where the parentheses indicate solid 475% Carbon' Solution oompounds- 40 8. The ternary alloy of claim 2 1n which there is about Three -other phase regions of potential interest in the 34% Chromium and about 4 75% C31b011- M-C-Cr systems are those defined below. The required 9- The tem3fy 31105 0f 6131111 1 1n Whlch the uranium, Chromium addition and fuel dilution are based on calchromium, and carbon compositions are within the area bon deviations of 10.1 w/o. Smaller deviations would A-BpD-E 0f FIGURE 4 0f the drawingsrequire proportionately smaller chromium additions. The 10- The ternary alloy of claim 1 in which the uranium, list of three phase areas of interest of the various systems chromium, and carbon compositions are on the line Z X are tabulated in the table. of FIGURE 4 of the drawings.

TABLE Weight, Systems Secondary percent; Secondary Phases Solidus, Phases,

or C. V/o 9 U UPu ThU ThPu Cmopius Ci- X X X X 1,315 6.4 CrvCa plus CrzaC... X X X X 1,530 8.3 CraCi plus Cr7Ca.... X X X X 1,680 11.1 MCrCz plus CrsCz.- X -6 WhereX denotes probable existence of three phase regions in system shown; denotes insuticient information.

The irradiation stability in these three phase regions with the exception of the region in which MCr2 is a secondary phase should be comparable to stoichiometric UC since the secondary phases are not ssile phases. The chromium additions appear to result in unique structures in this respect. Thermodynamic calculations and limited experimental results indicate that Cr23C6 is not attacked by sodium. The higher carbides of chromium are presumably less stable and carbon transfer should become increasingly probable at the higher carbon to chromium ratios in the carbides.

Accordingly, the present invention results in alloys having three phases which do not form low temperature eutectics with the cladding material, and have a lower tendency to carburize with the cladding because the sec- 11. A mixture containing, by Weight, 4.56 to 4.85% carbon, 0.40 to 5.17% chromium, and the balance essentially M, composed of intimately intermixed particles selected from the group consisting of chromium, carbon, and compounds and alloys thereof, wherein M, represents one metal selected from the group consisting of U, Pu, and UPu.

12. A mixture containing particles selected from the group consisting of M, chromium, carbon, and'compounds and alloys thereof, wherein M represents one metal selected from the group consisting of U, Pu, and UPu, and the composition of the mixture being Within the area A-B-D-E of FIGURE 4 of the drawings.

13. The mixture of claim 11 wherein the UPu alloy comprises from 15 to 30% by weight of plutonium.

14. The mixture of claim 11 in which the total chromium is about 3.4% by Weight.

1S. The mixture of claim 11 in which there is about 4.75% by Weight of total carbon.

16. The mixture of claim 11 in which there is about 4.75 by weight of total carbon and about 3.4% of total chromium.

17. The mixture of claim 13 in which there is 4.75% of total carbon.

18. The mixture of claim 3.4% of total chromium.

19. The mixture of claim 13 in which there is about 3.4% of total chromium and about 4.75% of total carbon.

20. A fuel element for a nuclear reactor comprising an elongated tube composed of a high temperature ferrous-base alloy, a plurality of pellets within the tube an-d having a composition of MC, Cr, and Cr23C6, wherein M denotes the heavy metal atoms of U, Pu, and UPu, the pellets having a diameter slightly less than and forming a clearance with the inner surface of the tube, and liquid metal occupying the clearance.

21. The fuel element of claim 20 in which there is from 4.56 to 4.85% by weight of total carbon and from 0.40 to 5.17% by Weight of total chromium.

13 in which there is about 22. The fuel element of claim 20 in which the uraniurn-plutonium alloy comprises from 15 to 30% plutonium.

23. The fuel element of about 3.4% chromium.

24. The fuel element of claim 20 in which there is about 4.75% carbon.

25. The fuel element of claim 20 in which there is about 3.4% of chrouimum and about 4.75% carbon.

claim 20 in which there is References Cited UNITED STATES PATENTS 2,526,805 10/1950 Carter et al. '75--122.7 X 3,041,260 6/1962 Goeddel 176-89 X 3,136,629 6/1964 Williams et al 75122.7 3,162,528 12/1964 Williams et al. 75-122.7 3,202,586 8/1965 Webb et al. 176-70 3,228,885 1/1966 Barta et al. 252--301-1 3,244,599 4/1966 Hildebrand 176--72 BENJAMIN R. PADGETT, Primary Examiner. CARL D. QUARFORTH, Examiner. M. J. SCOLNICK, Assistant Examiner. 

1. A TERNARY ALLOY CONSISTING ESENTIALLY BY WEIGHT OF 4.56 TO 4.85% CARBON, 0.40 TO 5.17% CHROMIUM, AND THE BALANCE CONSISTING OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF URANIUM METAL, PLUTONIUM METAL, AND URANIUM-PLUTONIUM ALLOY.
 11. A MIXTURE CONTAINING, BY WEIGHT 4.56 TO 4.85% CARBON, 0.40 TO 5.17% CHROMIUM, AND THE BALANCE ESSENTIALLY M, COMPOSED OF INTIMATELY INTERMIXED PARTICLES SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, CARBON, AND COMPOUNDS AND ALLOYS THEREOF, WHEREIN M, REPRESENTS ONE METAL SELECTED FROM THE GROUP ONSISTING OF U, PU, AND UPU.
 20. A FUEL ELEMENT FOR A NUCLEAR REACTOR COMPRISING AN ELONGATED TUBE COMPOSED OF A HIGH TEMPERATURE FERROUS-BASE ALLOY, A PLURALITY OF PELLETS WITHIN THE TUBE AND HAVING A COMPOSITION OF MC, CR, AND CR23C6, WHERE- 